- Email: [email protected]
Total efficiency of GE detectors—dead layer signal effect
ARTICLE IN PRESS Applied Radiation and Isotopes 68 (2010) 1451–1453
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Total efficiency of GE detectors—dead layer signal effect Pavel Dryak a,, Petr Kovar a, Arunas Gudelis b a b
Czech Metrology Institute, Radiova 1, CZ-102 00 Prague 10, Czech Republic Institute of Physics, Savanoriu ave. 231, LT-02300 Vilnius, Lithuania
a r t i c l e in fo
abstract
Keywords: Dead layer MCNP method Total efficiency HPGe detectors
Two types of detectors (GC 4018 and BE 5030) were compared regarding the signal from the outer dead layer of the detector. g-spectra of Am-241were acquired with various delays against the starting signal from an alpha LS probe. Coincidence spectra from both detectors were different. The dead-layer signal does not increase the full-energy peak efficiency but it increases the total efficiency, as it was demonstrated by the reconstruction of Am-241 spectra for both detectors. & 2009 Elsevier Ltd. All rights reserved.
1. Introduction During the study of the dependence of the GC 4018 type detector’s total efficiency (hereinafter referred to as et), differences were observed between the measured et values and et values calculated by means of the MCNP method (Drya´k and Kova´rˇ, 2006). Experimental values for energies up to 90 keV were higher by up to 50%; with energies exceeding 90 keV the difference decreased with increased energy down to several percent. We see the cause in the existence of a signal coming from energy absorbed in the dead layer, where absorption of low energies is significant and where a possible signal from the dead layer can contribute to et. This signal does not contribute to efficiency at the full-energy peak because it is reduced by the irreversible capture of charges in traps. It can also be delayed when captured charge in traps is re-released. The effect depends on the thickness of the dead layer. To study the effect, two types of Ge detectors were used in the experiments, a GC 4018 detector with a dead layer thickness of 0.53 mm and a BE 5030 detector with a dead layer thickness of ca. 500 nm. The experiment consisted of recording the coincidence gspectra of Am-241 as a function of the delay between the signal from the liquid scintillator probe containing Am-241 and the signal from the Ge detector.
2. Experimental setup For the measurement of coincidence spectra, the experimental setup was used as given in Fig. 1. Corresponding author. Tel.: + 420 26602 0497; fax: + 420 26602 0466.
E-mail address: [email protected] (P. Dryak). 0969-8043/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2009.11.026
The individual components are: 1—an adapted scintillation probe; 2—a pycnometer with LS and Am-241 LS; 3—a Canberra 2020 amplifier; 4—a TSCA Canberra 2037A discriminator; 5—a GC4018 or BE 5030 detector; 6—a Canberra 2024 amplifier; 7—a TSCA ORTEC 551 discriminator; 8—a Canberra 2040 coincidence circuit, resolution time 1 ms; 9—MCA: DSP 9660+ AIM 556A. During the measurement, the scintillation probe was apposed to the detectors as close as possible in order to achieve the highest efficiency of gamma detection. The efficiency of alpha detection was 99%. The delay in the a-channel was always fixed such as to enable the measurement of the whole coincidence curve. The delay of the g signal with reference to the a signal was adjusted over a range of ca. 0.7–4.5 ms. The coincidence signal was led to the GATE INPUT DSP. The spectrometric signal from the ENERGY OUTPUT of the GE detector preamplifiers was led to the INPUT DSP.
3. Coincidence measurements The measured coincidence curves are illustrated in Fig. 2 as the dependence of the total rate of impulses in the coincidence spectra relative to delay. A spectrum was taken for each a–g delay value and for both detector types. When a comparison is made between the coincidence curve shapes it is clear that the GC 4018 detector generates a nonnegligible number of delayed impulses whilst the BE 5030 detector generates none, or a much lower number. Fig. 3 demonstrates how the quality of spectra, expressed as a ratio of the 60 keV peak area to the total number of impulses, changes in relation to delay. It is clear that for the BE 5030 detector the quality is constant in range of 1–2 ms while for the GC 4018 detector the spectra quality decreases with delay.
ARTICLE IN PRESS 1452
P. Dryak et al. / Applied Radiation and Isotopes 68 (2010) 1451–1453
3
4
1
Input 6
9
N, rel
5
1000
Gate input
8
2
BE 5030
10000
7
100 10
Fig. 1. Connection diagram for the measurement of coincidence spectra.
1 0.0
coinc.curves 1.2
coinc.rate, rel
30.0
40.0
50.0
60.0
70.0
Fig. 5. Coincidence spectra for BE 5030, delays of 1.2 ms () and 2.1 ms (3).
0.8 0.6 0.4 0.2 0 0
1
2
3
4
5
delay, µs Fig. 2. Coincidence curves for detectors GC 4018 () and BE 5030 (3).
peak to total
20.0
keV
1
spectra quality
0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0
The spectra are different; the spectrum delayed by 2.25 ms is mainly formed by lower energies and we assume that this is mainly a signal from the dead layer. These delayed spectra are registered in the area below the full-energy peak and therefore they contribute to et. Fig. 5 illustrates two plotted coincidence spectra for the BE 5030 detector for delays of 1.2 ms (start of curve) and 2.1 ms (end of curve). The spectra were normalized so that the area of the 60 keV peak was the same. By contrast to the GC 4018 detector, the spectra within the statistics are identical. The analogue signal similar to the one from the GC 4018 detector dead layer is missing, and therefore there is no contribution of that type to the total efficiency.
4. Evaluation of coincidence measurements
0
1
2
3
4
5
delay, µs Fig. 3. Peak-to-total parameter of detectors GC 4018 () and BE 5030 (3).
GC 4018 10000 1000 N/100 s
10.0
The differences of time delays of signals from the two different detector types having different dead layers were measured. While the BE 5030 detector provides uniform spectra along the whole width of the coincidence curve, the GC 4018 detector provides spectra with different peak-to-total ratio dependent on the a–g delay. From the point of photon detection, the detectors in principle differ only in the dead layer thickness and for this reason we consider the existence of a signal from the dead layer as proved. The two detectors may behave differently with respect to charge collection because they have a different structure of the electrodes and of the electric field which can result in differences in signal rise time and in the charge collection time. The dead layer signal does not contribute to the full-energy peak efficiency, but contributes to the experimental total efficiency; this contribution is not included in MCNP. The reconstruction of the Am2141 spectra for both detectors emphasizes this contribution.
100
5. Reconstruction of Am-241 spectra by means of et calculated for both detectors
10 1 0.0
10.0
20.0
30.0
40.0
50.0
60.0
70.0
keV Fig. 4. Coincidence spectra for GC 4018, delays of 1.62 ms () and 2.25 ms (3)
In Fig. 4, two coincidence spectra for the GC 4018 detector are plotted, one for a delay of 1.62 ms (corresponding to the coincidence maximum of the coincidence curve) and for a delay of 2.25 ms (corresponding mostly to delayed impulses).
As evidence of an increase in et for the GC 4018 detector and non-increase in et for the BE 5030 detector against the calculated values, it is advisable to carry out a calculation of the total area of the Am-241 spectrum with the help of calculated et. The experimental spectra were measured with a standard with an activity of 87.06 kBq. The numbers of impulses for both detectors were set in intervals of 2–62 keV, corrected in the background. The working distances were always 25 cm from the cryostat surface. For each detector, a model for the MCNP program was created. The models represented the detector bodies with a cryostat, a source holder, a source and shielding. Radionuclide
ARTICLE IN PRESS P. Dryak et al. / Applied Radiation and Isotopes 68 (2010) 1451–1453
Am-241 was selected because the effect of increased et is significant for energy of 59.54 keV. Summation effects for a 25 cm distance are negligible. With the help of these models, et values were calculated for the energies of LX-ray, and for 26.34, 33.19 and 59.54 keV. In the same manner, contributions were calculated for higher energies up to the interval of 2–62 keV. Nuclear data were taken from Be´ et al. (2004) and Chu et al. (1999) in such a way that the total intensity of LX-ray came from Be´ et al. (2004) with relative intensities from Chu et al. (1999). The total area of the spectrum T was calculated according to the equation X A YðEi Þ et ðEi Þ t T¼ i
where A is the activity, Y(Ei) the yield of photons with energy Ei, et (Ei) the calculated total efficiency for energy Ei, and t the measurement time. The ratio of experimental and calculated total area q =Texp/ Tcalc was q= 1,66(5) for GC 4018 and q= 0,98(5) for BE 5030. It is evident that calculated et values can be used for a detector with a thin dead layer but not for a detector with a thick dead layer.
1453
6. Conclusions The existence of the signal from the dead layer of the detector was demonstrated. This signal increases the value of the experimental total efficiency mainly in the case of low energy photons when the absorption in the dead layer is comparable or higher than the absorption in the sensitive volume. The total efficiency cannot be computed reliably by the MCNP method for detectors with thick dead layer but MCNP gives reliable results for detectors with thin dead layer.
References Be´ M.-M. et al., 2004. Table of Radionuclides, Monographie BIPM-5. ¨ Chu, S.Y.F., Ekstrom, L.P., Firestone, R.B., 1999. Table of radioactive isotopes, The Lund/LBNL Nuclear Data Search, Version 2.0. Drya´k, P., Kova´rˇ, P., 2006. Experimental and MC determination of HPGe detector efficiency in the 40–2754 keV energy range for measuring point source geometry with source-to-detector distance of 25 cm. Appl. Radiat. Isot. 64, 1346–1349.
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Total efficiency of GE detectors—dead layer signal effect Pavel Dryak a,, Petr Kovar a, Arunas Gudelis b a b
Czech Metrology Institute, Radiova 1, CZ-102 00 Prague 10, Czech Republic Institute of Physics, Savanoriu ave. 231, LT-02300 Vilnius, Lithuania
a r t i c l e in fo
abstract
Keywords: Dead layer MCNP method Total efficiency HPGe detectors
Two types of detectors (GC 4018 and BE 5030) were compared regarding the signal from the outer dead layer of the detector. g-spectra of Am-241were acquired with various delays against the starting signal from an alpha LS probe. Coincidence spectra from both detectors were different. The dead-layer signal does not increase the full-energy peak efficiency but it increases the total efficiency, as it was demonstrated by the reconstruction of Am-241 spectra for both detectors. & 2009 Elsevier Ltd. All rights reserved.
1. Introduction During the study of the dependence of the GC 4018 type detector’s total efficiency (hereinafter referred to as et), differences were observed between the measured et values and et values calculated by means of the MCNP method (Drya´k and Kova´rˇ, 2006). Experimental values for energies up to 90 keV were higher by up to 50%; with energies exceeding 90 keV the difference decreased with increased energy down to several percent. We see the cause in the existence of a signal coming from energy absorbed in the dead layer, where absorption of low energies is significant and where a possible signal from the dead layer can contribute to et. This signal does not contribute to efficiency at the full-energy peak because it is reduced by the irreversible capture of charges in traps. It can also be delayed when captured charge in traps is re-released. The effect depends on the thickness of the dead layer. To study the effect, two types of Ge detectors were used in the experiments, a GC 4018 detector with a dead layer thickness of 0.53 mm and a BE 5030 detector with a dead layer thickness of ca. 500 nm. The experiment consisted of recording the coincidence gspectra of Am-241 as a function of the delay between the signal from the liquid scintillator probe containing Am-241 and the signal from the Ge detector.
2. Experimental setup For the measurement of coincidence spectra, the experimental setup was used as given in Fig. 1. Corresponding author. Tel.: + 420 26602 0497; fax: + 420 26602 0466.
E-mail address: [email protected] (P. Dryak). 0969-8043/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2009.11.026
The individual components are: 1—an adapted scintillation probe; 2—a pycnometer with LS and Am-241 LS; 3—a Canberra 2020 amplifier; 4—a TSCA Canberra 2037A discriminator; 5—a GC4018 or BE 5030 detector; 6—a Canberra 2024 amplifier; 7—a TSCA ORTEC 551 discriminator; 8—a Canberra 2040 coincidence circuit, resolution time 1 ms; 9—MCA: DSP 9660+ AIM 556A. During the measurement, the scintillation probe was apposed to the detectors as close as possible in order to achieve the highest efficiency of gamma detection. The efficiency of alpha detection was 99%. The delay in the a-channel was always fixed such as to enable the measurement of the whole coincidence curve. The delay of the g signal with reference to the a signal was adjusted over a range of ca. 0.7–4.5 ms. The coincidence signal was led to the GATE INPUT DSP. The spectrometric signal from the ENERGY OUTPUT of the GE detector preamplifiers was led to the INPUT DSP.
3. Coincidence measurements The measured coincidence curves are illustrated in Fig. 2 as the dependence of the total rate of impulses in the coincidence spectra relative to delay. A spectrum was taken for each a–g delay value and for both detector types. When a comparison is made between the coincidence curve shapes it is clear that the GC 4018 detector generates a nonnegligible number of delayed impulses whilst the BE 5030 detector generates none, or a much lower number. Fig. 3 demonstrates how the quality of spectra, expressed as a ratio of the 60 keV peak area to the total number of impulses, changes in relation to delay. It is clear that for the BE 5030 detector the quality is constant in range of 1–2 ms while for the GC 4018 detector the spectra quality decreases with delay.
ARTICLE IN PRESS 1452
P. Dryak et al. / Applied Radiation and Isotopes 68 (2010) 1451–1453
3
4
1
Input 6
9
N, rel
5
1000
Gate input
8
2
BE 5030
10000
7
100 10
Fig. 1. Connection diagram for the measurement of coincidence spectra.
1 0.0
coinc.curves 1.2
coinc.rate, rel
30.0
40.0
50.0
60.0
70.0
Fig. 5. Coincidence spectra for BE 5030, delays of 1.2 ms () and 2.1 ms (3).
0.8 0.6 0.4 0.2 0 0
1
2
3
4
5
delay, µs Fig. 2. Coincidence curves for detectors GC 4018 () and BE 5030 (3).
peak to total
20.0
keV
1
spectra quality
0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0
The spectra are different; the spectrum delayed by 2.25 ms is mainly formed by lower energies and we assume that this is mainly a signal from the dead layer. These delayed spectra are registered in the area below the full-energy peak and therefore they contribute to et. Fig. 5 illustrates two plotted coincidence spectra for the BE 5030 detector for delays of 1.2 ms (start of curve) and 2.1 ms (end of curve). The spectra were normalized so that the area of the 60 keV peak was the same. By contrast to the GC 4018 detector, the spectra within the statistics are identical. The analogue signal similar to the one from the GC 4018 detector dead layer is missing, and therefore there is no contribution of that type to the total efficiency.
4. Evaluation of coincidence measurements
0
1
2
3
4
5
delay, µs Fig. 3. Peak-to-total parameter of detectors GC 4018 () and BE 5030 (3).
GC 4018 10000 1000 N/100 s
10.0
The differences of time delays of signals from the two different detector types having different dead layers were measured. While the BE 5030 detector provides uniform spectra along the whole width of the coincidence curve, the GC 4018 detector provides spectra with different peak-to-total ratio dependent on the a–g delay. From the point of photon detection, the detectors in principle differ only in the dead layer thickness and for this reason we consider the existence of a signal from the dead layer as proved. The two detectors may behave differently with respect to charge collection because they have a different structure of the electrodes and of the electric field which can result in differences in signal rise time and in the charge collection time. The dead layer signal does not contribute to the full-energy peak efficiency, but contributes to the experimental total efficiency; this contribution is not included in MCNP. The reconstruction of the Am2141 spectra for both detectors emphasizes this contribution.
100
5. Reconstruction of Am-241 spectra by means of et calculated for both detectors
10 1 0.0
10.0
20.0
30.0
40.0
50.0
60.0
70.0
keV Fig. 4. Coincidence spectra for GC 4018, delays of 1.62 ms () and 2.25 ms (3)
In Fig. 4, two coincidence spectra for the GC 4018 detector are plotted, one for a delay of 1.62 ms (corresponding to the coincidence maximum of the coincidence curve) and for a delay of 2.25 ms (corresponding mostly to delayed impulses).
As evidence of an increase in et for the GC 4018 detector and non-increase in et for the BE 5030 detector against the calculated values, it is advisable to carry out a calculation of the total area of the Am-241 spectrum with the help of calculated et. The experimental spectra were measured with a standard with an activity of 87.06 kBq. The numbers of impulses for both detectors were set in intervals of 2–62 keV, corrected in the background. The working distances were always 25 cm from the cryostat surface. For each detector, a model for the MCNP program was created. The models represented the detector bodies with a cryostat, a source holder, a source and shielding. Radionuclide
ARTICLE IN PRESS P. Dryak et al. / Applied Radiation and Isotopes 68 (2010) 1451–1453
Am-241 was selected because the effect of increased et is significant for energy of 59.54 keV. Summation effects for a 25 cm distance are negligible. With the help of these models, et values were calculated for the energies of LX-ray, and for 26.34, 33.19 and 59.54 keV. In the same manner, contributions were calculated for higher energies up to the interval of 2–62 keV. Nuclear data were taken from Be´ et al. (2004) and Chu et al. (1999) in such a way that the total intensity of LX-ray came from Be´ et al. (2004) with relative intensities from Chu et al. (1999). The total area of the spectrum T was calculated according to the equation X A YðEi Þ et ðEi Þ t T¼ i
where A is the activity, Y(Ei) the yield of photons with energy Ei, et (Ei) the calculated total efficiency for energy Ei, and t the measurement time. The ratio of experimental and calculated total area q =Texp/ Tcalc was q= 1,66(5) for GC 4018 and q= 0,98(5) for BE 5030. It is evident that calculated et values can be used for a detector with a thin dead layer but not for a detector with a thick dead layer.
1453
6. Conclusions The existence of the signal from the dead layer of the detector was demonstrated. This signal increases the value of the experimental total efficiency mainly in the case of low energy photons when the absorption in the dead layer is comparable or higher than the absorption in the sensitive volume. The total efficiency cannot be computed reliably by the MCNP method for detectors with thick dead layer but MCNP gives reliable results for detectors with thin dead layer.
References Be´ M.-M. et al., 2004. Table of Radionuclides, Monographie BIPM-5. ¨ Chu, S.Y.F., Ekstrom, L.P., Firestone, R.B., 1999. Table of radioactive isotopes, The Lund/LBNL Nuclear Data Search, Version 2.0. Drya´k, P., Kova´rˇ, P., 2006. Experimental and MC determination of HPGe detector efficiency in the 40–2754 keV energy range for measuring point source geometry with source-to-detector distance of 25 cm. Appl. Radiat. Isot. 64, 1346–1349.